This invention relates to reduction of the visibility of line-crawl artifacts in a progressively-scanned television display generated from a field-rate interlaced signal.
Attention has recently been directed to high-definition television (HDTV) systems, in which the displayed picture has great horizontal and/or vertical resolution for improved appearance on large-screen and projection-type displays. Because such systems may undesirably not be compatible with current color television standards, consideration has been given to improving the appearance of the display of conventional standard-definition television, such as NTSC or PAL, to produce an "enhanced-NTSC(-PAL)" system, requiring no basic change to current broadcast standards. Among the schemes proposed for enhanced-NTSC(-PAL) is a change in the receiver from interlaced display to progressively-scanned display. In the case of NTSC this involves starting with an incoming television signal representing 2621/2 raster scan lines, occurring in a field interval of 1/60 second, followed by a second set of 2621/2 raster scan lines interlaced with the first set, the second set also having 1/60 second elapsed time to form a "monochrome" frame of 525 lines in 1/30 second. The term "monochrome" refers to the fact that the 1/30 second frame does not include a complete repetition cycle of the phase of the color subcarrier relative to horizontal sync; an interval with a complete subcarrier phase repetition is termed a "color" frame and requires two monochrome frame intervals (1/15 sec) for 60 Hz, 525-line NTSC and four frame intervals (1/6.25 second) for 50 Hz, 625-line PAL.
To form a progressively-scanned display from a signal representing an interlaced scan-line raster, additional raster scan lines must be provided during each field. The signals representing each field can be delayed for a field interval and displayed in proper sequence with the lines of video of the next following field. In effect, the line of the preceeding field is actually inserted between lines of the current field. This scheme has the advantage of reducing flicker, motion-related artifacts, and line-crawl artifacts, but has the disadvantage of requiring a field store for storing each of the lines of a field for one field interval. Such field stores are expensive and consume substantial power.
One way of increasing the number of lines in a television field is to simply replicate each horizontal line, as described in U.S. patent application Ser. No. 359,612 in the name of R. A. Dischert (U.S. Pat. No. 4,415,931), which method requires line storage and simple electronics.
It is also known to use line-stores for delaying each line of incoming signal for a duration sufficient to perform an interpolation to generate signals representing raster lines interpolated between the raster lines of the current field. This is most simply accomplished by averaging the signals of two adjacent lines of a field to form by linear interpolation a signal representing an estimate of the signal representing the interlaced line of a temporally adjacent field. These estimated signals are simply inserted between unaltered lines of the current field. It is also known to use quadratic interpolation using more than one-line store, all as described in U.S. patent application Ser. No. 300,227, filed Sept. 8, 1981 in the name of K. H. Powers (now U.S. Pat. No. 4,400,719. When an interpolation scheme is used to estimate alternate lines to be displayed so that a double-rate progressively-scanned display may be operated from a source of interlaced video signals the term "pseudo-progressive" scanning may be used. Pseudo-progressive scanning eliminates line-crawl artifacts and reduces motion-related artifacts and flicker. It can also result in the loss of vertical spatial detail.
FIG. 1a illustrates in perspective view an interlaced raster having 525 scanning lines, only a few of which are shown for improved clarity. Scanning of the raster begins at the top left edge of the raster with line 1 which scans to a point 11 during one line interval and retraces to point 2 to begin a second scan which ends at point 12. The scanning continues with a succession of 262 scan lines of the first field. As illustrated, scan line 262 ends at point 14 at the right of the raster. The first field ends with the scanning of a half-line of line 263. Scanning of the second field begins with scanning of the second half of line 263, which ends at point 15. The scan retraces to the left and scans lines 264-525, which are interlaced or scanned between the lines of the first field. Scan line 525 ends at point 16. Scanning then begins again with line 1 in a recurrent manner.
FIG. 1b illustrates the recurrent scanning of FIG. 1a expanded to include the time dimension. As illustrated in FIG. 1a, the first field is scanned by 2621/2 lines at a time T0. At a time T2 which is 1/60 second (FOR NTSC) later than T1, the last half of line 263 is scanned and ends at point 15. The second interlaced field is completed by scanning lines 264-525. Scanning begins again at a time T3 which is 1/30 second later than time T1, with a scan corresponding to that of field one (1). This sequence recurs continuously, so that the field pattern illustrated in FIG. 1b continues indefinitely to the right.
FIG. 2 is a space-time representation of the raster lines of FIG. 1. FIG. 2 may be interpreted as a view along the x-axis of the representation of FIG. 1b. In FIG. 2, the scan lines are seen end-on and are represented by dots. The scan lines of the odd fields are illustrated by solid dots and the scan lines of even fields by open dots, as in FIG. 1. The time between successive fields is shown as 1/60 second, but could also be 1/50 second or any other interval. The vertical (Y-direction) spacing between a scan line and the adjacent scan line of the following field as illustrated is S, and 2S is the vertical distance between the location of a scan line of one field and the adjacent scan lines of the same field when the adjacent scan line of the next field is half-way between the scan lines of the current field.
FIG. 3a represents a Fourier transform designated generally as 300 of the vertical-direction, space-time representation of FIG. 2. The abcissa is measured in terms of inverse time or temporal frequency (ft), and the ordinate in terms of inverse distance or spatial frequency (fy). Spatial frequency is measured in cycles per picture height (cph) which for a particular screen size viewed at a particular distance translates into cycles per degree of subtended angle (cycles/degree) as described in the article Modeling the Human Visual System by Adelson, et al., published in Volumn 27, No. 6 of RCA Engineer, Nov./Dec. 1982. The desired signal in the vertical direction at any given time (i.e., with time a constant) is sampled by the raster scan lines with spacing 2S as illustrated in FIG. 2. Consequently, in accord with the Nyquist criteria the signal component at f.sub.t =0 Hz in FIG. 3a can extend only as far in the vertical direction as fy=.+-.1/2S. Those portions of the signal at spatial frequencies centered about multiples of .+-.1/2S are repeat spectral information resulting from sampling at 2S. These terms represent visible, undesirable artifacts in the image. For example, at 0 Hz (i.e., for a constant picture) the raster line structure is represented as an artifact at .+-.1/S. Another artifact can be understood by imagining a non-scanned white image field which flashes on and off every 1/60 second. This artifact is represented by points on the transform of FIG. 3a at f.sub.t =.+-.60 Hz along the frequency axis, fy=0. This artifact is known as large area flicker. There is another discrete artifact, which has both spatial frequency and temporal frequency components, and it is located at the points f.sub.t =.+-.30 Hz, fy=.+-.1/2S. This artifact is known as line crawl, and arises due to the interlace of the lines of successive fields. Physically, this may be understood by considering what happens if an eye 210, located as illustrated in FIG. 2, scans the display in a vertical direction. At certain eye scan rates, the successively arriving raster scan lines will be perceived as moving vertically. The line crawl results from psychovisual perception of the lines as moving in space as a function of time, and results from the eye following a space-time path such as is illustrated by chain line 212.
The pattern of FIG. 3a represents the transform of components as they would appear for a white raster scanned interlaced at 60 Hz, as suggested by raster 340 of FIG. 3b. The spectrum of a raster having a black-to-white transition is illustrated as 350 in FIG. 3c. The transition (illustrated in FIG. 3d) causes sidebands or spectral components to extend in the f.sub.y direction, as illustrated by dotted lines 356. These vertical components represent line or edge flicker. If the edge or transition between block region 352 and white region 354 of the raster of FIG. 3d moves, the motion causes spreading of lines 356 to fill with general motion artifacts those quadrangles of spectrum 350 in which the edge flicker components reside. For ease of understanding, the components of FIG. 3c are represented in a perspective view in FIG. 3e, with the amplitude axis illustrated at right angles to the f.sub.y and f.sub.t axes.
FIGS. 4a-h are illustrations aiding in understanding the notation used for interpolating filters. In FIGS. 4a-h, the abscissa represents the vertical direction measured in raster lines. An arbitrary image-representative signal 410 is shown having a value of 1.0 units in the range from 1 to 3 raster lines and a value of 2.0 units in the range from 6 to 8 raster lines and beyond, with a smooth transition in between. The signal only exists at the raster line points, as indicated by the dots. This signal may be imagined as a raster which is black (low signal level) at the top (raster lines 1-3) and white (high signal level) at the bottom (raster lines 6-8) with a transition in the region of raster lines 3-6. FIGS. 4b-4g represent some of the successive positions assumed by the response of a 3-tap interpolating filter (i.e., linear interpolation) which is convolved with or which scans the signal in time, and therefore in effect scans in vertical space. The filter response includes three peaks 412, 414 and 416 separated from one another by spatial distance S, equal to half a scan line separation. Peak 414 has a "multiplier" or "value" of 1/2 or 0.5, while "peaks" 412 and 414 each have a value of 1/4 or 0.25. The values of the multipliers of the various peaks in the spatial-frequency response of the filter are selected or normalized to form a sum value equal to unity, so that the intensity of the picture is the same before or after interpolation of lines, otherwise doubling the number of lines would double the brightness. As the filter receives the image-representative signal 410, the filter response effectively scans in space. At the instant illustrated in FIG. 4b, filter response peak 414 having a value of 0.5 coincides with raster line 1, while peaks 412 and 416 do not coincide with a raster line. The value of the signal produced by the filter at any moment of the scan is established by multiplying the value of the signal intercepted at that moment by each filter peak by the multiplier associated with that peak or tap, and then summing those weighted values. For the filter position represented by FIG. 4b response peak 414 intercepts a signal having a value of 1.0 and response peaks 412, 416 intercept zero signal. The value of the filter output signal in the position illustrated in FIG. 4b is: EQU (0.25.times.0)+(0.5.times.1.0)+(0.25.times.0)=0.5.
The above value produced by the filter is plotted as 420 as illustrated in FIG. 4h. The filter continues to scan, assuming the position illustrated in FIG. 4c one-half raster scan later. In this position, filter response peak 414 does not intercept any signal, while response peaks 412 and 416 each intercept signal having a value of 1.0 at raster scan lines 1 and 2, respectively. The value of the filter output is computed EQU (0.25.times.1.0)+(0.5.times.0)+(0.25.times.1)=0.5
which is plotted as 422 in FIG. 4h. The filter continues to scan, successively taking on vertical positions, some of which are illustrated in FIGS. 4d-g. An output signal is produced twice during each traversal of a distance 2S. One such output signal occurs when the central response peak 414 intercepts the signal value at a raster scan-line, and the other occurs when peak 414 is half-way between raster scan lines, at which time response peaks 412 and 416 intercept the adjacent raster lines. When filter response peak 414 intercepts signal, the output signal is proportional to the actual intercepted signal. When filter response peaks 412 and 416 intercept signal, the filter output signal is proportional to the average of the two signals intercepted. In FIG. 4, the value of the product of each illustrated response peak times the intercepted image 410 is noted below the response peak. The filter output signal represents actual raster lines interspersed with raster lines interpolated by averaging. Other filter responses can be represented in the same fashion, by a succession of spaced response peaks, and the output of the filter is established as the sum of the various instantaneous products of the response peak value multiplied by the intercepted signal value.